1. Field of the Invention
The present invention relates to the alignment of a crystalline axis of an optical device having Brewster faces; and, more particularly, to aligning a crystalline axis of a birefringent element so that it lies in the plane of the polarization defined by Brewster faces of the birefringent element.
2. Description of Related Art
Optical devices having Brewster cut faces are in widespread use. Brewster faces define a plane of polarization for light propagating though the optical device. Many such devices are fabricated from crystalline material with well-defined crystalline axes. Such optical devices are manufactured so that one such crystalline axis is as close to the direction of polarization as possible. However, with current manufacturing techniques, it is difficult to ensure that the selected crystalline axis is closely aligned with the direction of polarization.
Where the optical device is birefringent and defines a long path between the Brewster faces, this misalignment of the crystalline axis can result in significant depolarization effects. When such an optical device is used in a laser cavity that includes another element which is sensitive to polarization, the depolarization effects can have an effect on the performance of the sensitive element. For instance, birefringent filters in widespread use in tunable lasers are very sensitive to the polarization of the beam in the cavity. Background concerning birefringent filters can be found in Bloom, "Modes of a Laser Resonator Containing Tilted Birefringent Plates", JOURNAL OF THE OPTICAL SOCIETY OF AMERICA, Vol. 64, No. 4, April 1974; Preuss et al., "Three-Stage Birefringent Filter Tuning Smoothly Over the Visible Region: Theoretic Treatment and Experimental Design", APPLIED OPTICS, Vol. 19, No. 5, Mar. 1, 1980; Holtom et al., "Design of a Birefringent Filter for High-Power Dye Lasers", IEEE JOURNAL OF QUANTUM ELECTRONICS, Vol. QE-10, No. 8, August 1974; Mudare et al., "Simple Alignment Procedure for the Assembly of Three-Plate Birefringent Filters for Tunable Dye Lasers", APPLIED OPTICS, Vol. 22, No. 5, March 1, 1983; and November et al., "Derivation of the Universal Wavelength Tuning Formula for a Lyot Birefringent Filter", APPLIED OPTICS, Vol. 23, No. 14, July 15, 1984.
One of the factors which limits the smooth tuning range of tunable solid state lasers, such as those using a Ti:sapphire or a cobalt magnesium fluoride gain medium, is the misalignment of the selected crystalline axis of the gain medium with resulting depolarization, affecting the operation of the birefringent tuning filter. As recognized in Schulz, "Single-Frequency Ti:Al.sub.2 O.sub.3 Ring Laser", IEEE JOURNAL OF QUANTUM ELECTRONICS, Vol. 24, No. 6, June 1988, it is very important to ensure that the crystallographic orientation of the Brewster faces aligns the C-axis of the Ti:sapphire crystal as close as possible to the direction of polarization of the laser beam. However, due to the relatively long optical path through the gain medium, the effect of a small misalignment can be significant.
Accordingly, it is desirable to have an apparatus for minimizing the depolarizing effect of misalignment of the optic axis of birefringent elements with respect to Brewster cut faces of those elements. Further, it is desirable to make such an alignment without disturbing the operation of the laser cavity.